System for the neutralization of inductive interference



May 21,- 1935. LOCKROW v ,8

SYSTEM FOR THE NEUTRALIZATION 0F INDUCTIYE INTERFERENCE Filed April 15, 1953 Ground conn ecf/an f0 Shea Connecfion to sheaf/1 .Pawer Conductor llanflectllolzs to cable sheath INVENTOR LLLOakI 0w BY W ATTORNEY- Patented May 21, 1935 UNITE ST ES I SYSTEM FOR INDUGTIVE INTERFERENCE .i 1

Laurlce Laird Lockrow, Br oklyn, N'; 1., assignor t to American Telephone and Telegraph Company, a corporation of New York Ap cation April is, 1933, Serial mmerse 9 Claims. (c1, its-78v v This invention relates to electrical circuits and.

more particularly, to circuits for the neutralization and suppression of induced voltage audits effect. Still more particularly, this invention relates to circuits employing neutralizing transformers which may be applied to groups of electrical conductorsand, moreover; it relates to neutralizing transfonners which may be applied to cables and cable sheaths enclosing groups of. conductors.

Neutralizing transformers have heretofore been employed to reduce the interference in communia cation circuitswhich maypbe produced bynearby power lines or other; disturbing circuits to'which the communication circuitsjare exposed. The use p of neutralizing transformers generally involves an auxiliary circuit in which are induced the same disturbances as are induced in l the. communication circuits. In suchlarrangements, the neutral? izing transformers have their primarywindings connected in the auxiliary circuit and their secondary windings in the communication circuits, the secondary windings beingso related to theprimaries that the voltages induced therein by the primary windings wilhtend to oppose those induoed directly by the power or disturbing circuits into the communication circuits. i

Various arrangementshave been suggested from time to time for constructing these primary and secondary windings with a view to improving the couplingbetween them in order to suppress the induced effects more perfectly, but because ofthe complexity of the suggested arrangements they were generally impractical. Theseqformer arrangements generally involved the use of primary andsecondary windings which were wound around iii-common magnetic iron core, the primary windings being connected in series relation with the auxiliary circuit and the secondary windings in series relation with'the communication circuits to berpmtected. L

\ Inthis connection it will be noted that the primary and secondary windings. above mentioned have a considerable amount of mutual capacity and direct current-resistance and certain of the designs that have been-suggested in the past also add an appreciable amount of inductance tothe communication circuits. In consequence these neutralizing transformer wind ngs cause an Qbzlec+ tionable transmission loss in the communication circuits in which they are connected andyalso cause substantial impedance irregularities. These cuits.- These various would tend to a be particularly objectionable in multi transtormer installations and in some cases their cumulative effect would become intolerable.

This inventionrproposes to reduce the various impairments mentioned above by designing a transformer suitable for the neutralization and suppression of induced voltage effects, said transformer of a magnetic iron core which surrounds the ground electrical conductors to be protected, or, more particularly, a communication cable consisting of a cable sheath enclosing groups of conductors Another object of this invention is toprovide a simple and comparatively inexpensive means-of obtaining the equivalent of a primary winding in cable circuits or; in other groups of conductors which will give a maximum of magnetic coupling between the element forming the equivalent of said winding and another element or elements forming the equivalent of a secondary winding or windings, these elements together forming a transformer employed f-or the neutralization and suppression of induced voltage efiects.

- Still another object of my invention is to promechanical and electrical disturbance to the communication circuits or groups of electrical conductors to be protected is practically My invention is illustrated in the accompanying drawing in which Figure 1 shows the application, of a special type of neutralizing transformer to a eommunication cable and its enclosed group of conductors for the neutralization and suppression of induced voltage effects in said group of conductors. Figs. 2 and 3 are equivalent circuit used in explainingihc invention. Fig.

4 shows the manner of arranging additional shunt so ,as to make theitransformer efiective at several frequencies Fig, 5 shows analternate of connecting a tuning condenser across the equivalent of the primary winding of the special neutralizing transformer involved in] invention. I r I -In Fig. 1 is indicated thecommunication cablel- -l having its metal sheath 2-2 grounded at tile points 3-}, metalsheath encloses a group of separately insulated wires designated 44. The cableis e i sed to a power .lineo'r a other disturbing circuit or designated I'|, and it will be understood that the disturbing circuit or circuits may have any well-known form. 1

The arrangementshown in Fig. 1 consists of an iron core 5 constructed of circular laminations placed around the cable and stacked lengthwise along it. A number of turns of, a'windingdesignated 9 are wound around the core, these turns being insulated from eachother and from the core 5. It will beunderstood' that this'insulation may be obtained by wrapping the winding 9 in any well-known form of insulating material, thereby reducing the necessa1y spacings to a minimum, or the turns of said windingmay be so spaced from each other and from thecore 5 as to be-definitely insulated from each other. A condenser designated I0 is' connected to the extremities or terminals of the winding 9 for adjusting the phase relation-between the neutralizing voltage set up in the special neutralizing transformer of this invention and thedisturbing voltage. The "section of the cable sheath between the points designated I'I-l I which are immediately adjacent to the opposite-faces of the core 5 serves as the equivalent of the primary of the transformer and the conductors enclosed within the section ofsheath designated II--I I serve as theequiv'alent of the'secondary windings of the transformer and these are designated I2- I2'. It will be noted that the conductors I2-I2 which form the secondaries of the transformer and which are enclosed within the section of the cable sheath II'--II are, in fact, portions of the conductors 4-4.

Since the tuning condenser I0 is connected across the winding 9 which is wound around the iron core 5, it is connected inductively to the section of cable sheath designated IIII which serves as the primary of the transformer. The auxiliary circuit of this neutralizing transformer consists of the cable sheath 2-2 and the grounds at points 3 -3 which are connected to the cable sheath at, for example, the ends of the power exposure. Although the auxiliary circuit of this neutralizing transformer is shown as consisting of the cable sheath 2--2 and the grounds at points 3-3, it will be understood that the auxiliary circuit may, if desired'consist of ground points such as 33 and the-metal sheath 2--2 and other conductors connected'in parallel with said metal sheath such as conductors enclosed within said sheath and/or conductors not enclosed therein but parallel and adjacent thereto. In cases where any two of the conductors such as 44 (or any multiples; of two conductors) are used in parallel with the metal sheath as the auxiliary circuit, these conductors can be connected to ground points such as 3-3 through a retardation or drainage coil, thereby allowing the use of these conductors for communication purposes.

Since the cable sheath 2-2 and the electrical conductors 44 which areenclosed therein go through the iron core 5 as a'sing'le' unit, each of the secondaries designated I 2--I2 will have the same number of turns as the cable sheath I II I which serves as the primary of the transformer.

,Since, in' the case of communication circuits exposed to induction, as, for example, induction from power lines or electricalrailways or the like,- the value of induction is not ordinarily steady but varies from timeto time through'wide limits due to wide variations in disturbances on said power lines or the like, oneor'more air gaps such as 8 are placed in the'magnetic core 5 so as sible, but it is possible tochoose the constants of to make the reluctance of this special type of transformer more constant through the range of flux densities for which this transformer may operate. This also prevents saturation of the iron core, thereby rendering the induction due to transformer action practically proportional to the inducing force such as that caused by the neighboring power lines. It is not practicable to use a condenser as a phase shifting device in conjunction with a winding located on a core which does not have an air gap because a core of this type has variable reluctance and the winding therefore has variable inductance corresponding to variations in the magnitude of the induced voltages. However, because of the air gaps designated 8, the transformerpf this invention has practically a constantvalue of inductance through a wide range of flux densities, thereby permitting the effective use of a shunt condenser such as ID for controlling the phase relation between the voltage set up in the transformer and the disturbing voltage.

The magnetic iron core 5 of the transformer of Fig. l is divided into two portions by the air gaps designated 8. These two core portions may be adjustably held in proper spaced relation by suitable clamping means (not shown) in order that the length of gaps 8 maybe adjusted to the proper value. An increase in the air gaps 8 produces a decrease in the inductance of the wind ing 9. This is compensated for by increasing the capacity of the shunt condenser i0. justment in the air gaps 8 assist in obtaining the value of inductance which best suits the particular conditions applying for a given transformer. By this particular transformer arrangement, the neutralization and suppression of the induced voltage efiects are obtained by transformer action, said transformer action causing a substantial increase in the self-impedance of the cable sheath circuit iI-II which serves as the primary and in the mutual impedance between this primary and the cable conductors I2I2 which serve as the secondaries.

The action of this transformer in increasing the self-impedance of the section of the cable sheath between the points IIII or, in other words, the primary, is best described in terms of the equivalent circuit of Fig. 2. Referring to this figure, the section of the cable sheath between the points designated IIII constitutes the primary of the transformer and the winding 9 which is wound around the iron core 5, the secondary. The tuning condenser I0 is connected across the terminals of the secondary winding 9. By the usual circuit analysis the self-impedance of the section of the cable sheath between the points IIII becomes:

where The terms Z, Z1, Z2, ZM and Z0 are shown in their relative positions in the equivalent circuits of Fig. 2.

It it were possible to make Z2:Zc, the impedance Z would become infinite. This is not pos- Thus, ad-

the following expression:

lagoons-iv Duetothe'fact that the some sheatnscrrouncs the cable conductors and that the iresistance of ,the sheath between 311% points designated i I I is very small, the mutual impedance between the cable sheathoireuit I l and the grouper conductors enclosed therein is substantially the same as the sen-impedance or the cable slieeth en I It follows then that for ahyinc'rcase in the self impedahce ol'tne sheath circuit 14 -11 between the sheath oileoituesignated 1 1 and the group or conduetois therein, these conductors beingdesignsted 12- -1! a d being a porno 11 f the conductors designated 1-4.

'Ifhe action of the entire circuit is best de in terms of the equivalent circuit of Fig. 3 m which.,:.? .1 E=v'oitage ihduced intne sheath and its fenclosed conductors by the earth-return power: of conductor currenu. l

V=net,- voltage in each cable conductor as ine'asured by high reslstance voltmeter.

Re resistanceor each round connection or the auxiliary EfifQHitwt a zs=tota1gseli4mpedance of sheath circuit-ineluding grounds but exclusive o1. that due to transformer action. V

R effecti've resistance or metal sheath. zn=earth-return mutual-impedance of the sheath to its enclosed conuuctorspexelusiw of that due to transformer action; I V

Z=seli"-"mp'edance of sheath due to transformer action which is also equalfto mutualimpedance or the sheath :to its enclosed conductors due to trans-tonne:- action.

The current I in the sheath circuit is given by p fz+ z. The net voltage v in the cable conductor is given by the following expression:

And the ratio of the net voltage' to the voltage inducedfrom the power circuit is given by the the latter ratio reduces to the ,following eirpression: K .R+ Ra I -E 2+2.z A 1 From the latter expression it appears that the larger Z can be made, thelower the ratio becomes. g l i v It is possibleby proper choice of the circuit constants Z2 and Z0, as shown in Fig. 3, to. obtain a tuned circuit such as A, this circuit having a maximum impedance atthe critical .or fundamental frequency of the induced voltageas preyiously described in the explanation of 2. Z2 represents the impedance ofwindin'g' iwhile fie represents "the im edance of the tlm i-h'g con denser lo. Thus, byv-irtue of this tuned circuit and the transformer action, it can be seen that the impedance z of the cable sheath becomes very large at the fundamental frequency of the induced voltage. The large valuc of- Z, there- 2;,

fore, makes-therati'o very small and this in turn indicates a very large reduction indie original induced voltage, this reduction being so great that the net voltage approaches zero. f

Where induced voltages of several. different frequencies are present as, for example; the fundamental togetherwith the third and fifth harmonics, it is possibleto neutralize and sup press these voltages by providing additional shunt circuits consisting of elements i8, I1, etc.,-=as shown in Fig. 4. These shunt circuits will give the. proper phase relation between the neutraliz ing and induced voltages at these different frequencics. Itwill be noted that these shunt circuits areconnected across winding 9,'an indi, vidual shunt circuit being provided; for, each additional frequency. 4; 6 Instead of employinga winding 9 wound around a. magnetic core, with a tuning condenser-1B connected across the terminals of that winding, as shown in Fig. 1, the tuning condenser 10 may be connected to the cablesheath at points yl l-ll through an iron core transformer designated as 43, as shown in Fig. 5. 'The primary winding [4 'of this transformer is connected to thefcable sheath at; points Il -H and the secondary winding l5. to the tunlngcondenser l0. .lt will be understood that this transformer may have any suitable turnratio which permits the use of condenser of convenient size. As discussed above additional shunt circuits designated as ,l 6 and. i1. may be connected across thesecondary winding l5 so as to obtain proper phase relation between the neutralizing and induced voltages atother frequencies as well assat the fundamental. Although the condenser I8 is shown as beinginductively connected to thcmetal sheath'through the transformer l3, it will be understood that the condenser l0 may, if desired, be connected directly to the metal sheath at points such as I I'| Also, it will be understood'that the primary of this transformer designated here as thenmetal sheath circuit l l--| I'may consist of a metal different from that generally used in communication cables and, more particu1arly,-a metal sleeve or sheath of higher conductivity than that used in commercial cables. This may be desirableincases where the resistance of the cable sheath itself may be so high as to preclude the proper phase relation of the neutralizing and inducedvoltages.

In cases where the induced voltage is'cqual to or less than the rated voltage of the transformer, only one transformer and two ground connections to the auxiliary circuit arerequired, the ground connections being made at the ends of thB PDWer exposure. Where the induced voltage is greater than the transformer rated voltage, scveraltransformers are placed along the cable, the ground connections tothe auxiliary circuitheirig pro vided at the ends of the powerex'posure and at variouspoints within the exposure. These latter ground points are so spaced and located between the transformers as to divide the total induced voltage into parts whose magnitude is equal to or less than the rated 'voltage 'of one transformersaid transformer being pIacedmidWay, in 16 111 duced voltage scale, between consecutive ground points. j

The operation of this transformer isas follows: The auxiliary circuit composed of the cable sheath 2-2 and the grounds at points '33 has induced in it from the disturbing circuit 1'| the same voltage as the group of conductors 4-4 enclosed within the sheath. Through transformer action at the fundamental frequency of the induced voltage as, for example, frequencies such as 25 or 60 cycles, winding 9 with its shunt condenser I causes a substantial increase in the self-impedance of that section of the cable sheath between points I l| l and consequently a substantial increase in the mutual impedance between that section of cable sheath designated H-ll and the section of cable conductors designated l2-I2, this latter section of conductors being a portion of the cable conductors 4.4.' Therefore, the self-impedance of the cable sheath between points l|ll at the fundamental frequency of the induced voltage willbe large when compared to the self-impedance of the auxiliary circuit consisting of the-remainder of the cable sheath '22 and the grounds at points '3-3, in which case practically all of the voltage induced in the auxiliary circuit 33 and 2+2 will appear across the section of the cable sheath between lll I. By virtue of the high mutual impedance between the section of sheath designated H'I| and the section of conductors designated |2- i2, .this voltage will appear in the cable conductors between points l2-l2. By virtue of proper phase relationbetween this neutralizing voltage, and the induced voltage obtained by using the shunt condenser Ill, the polarity of this. neutralizing voltage is such as'to oppose that voltage induced directly in the cable conductors 4-4 by the disturbing circuit or circuits l"i.' Thus the net longitudinal voltage appearing in the cable conductors 44 will be relatively small and will approach zero.

While the circuits l6 and I I of Figs. 4 and 5 may be employed in conjunction with the con-'- denser H! in an arrangement for the suppression of a 25'or 60 cycle voltage or any other voltage and certain 'of its harmonics, for example, the 3rd and 5th harmonics, as described hereinabove, it will be understood, however, that the circuits l6 and I! need not function for the suppression of harmonics of a fundamental frequency but that these may be employed to suppress any frequencies thatmay interfere with transmission over the conductors 44 and that the frequencies suppressed by the circuits I6 and i! need not be related in any manner to the frequency suppressed by the coil 9 and condenser. l0.

While the arrangement shown in Figs. 1, 4 and 5 are specifically directed to an aerial cable sheath and its enclosed conductors, it will be understood that the principles of this invention may'readily be applied to any other form of cable sheath and its enclosedconductcrs. If the cable sheath were located under ground, it" would bedesirable either to insulate the-cable sheath from ground throughout the lengthof its exposure except'at the grounding points which occur approximately at the ends of the exposure; or, if preferred, the sheath may be continuously grounded and a plu-v rality of similar devices {each including a .core such as 5 and a Winding such as 9 may be located along the sheath at spaced intervals. In the latter case, each winding- 9. and its corresponding condenser I0, may be considered asa tuned circuit which is resonant at the frequency of the current which it is intended to suppress or neutralize.

While I have shown and described certain forms of my invention merely for the purpose of illustration, it will be understood that various modifications may be made without departing from the spirit and scope of the appended'claims.

What is claimed is:

1.."A transformer arrangement for neutralizing the effect of induction from exposure to power circuits comprisinga cable sheath; a plurality of conductors insulated from each other and enclosed Within the cable sheath, the cable sheath being grounded at the ends of the section exposed to the power circuits, a core-positioned coaxially about the cable sheath substantially at the midpoint of the voltage eil'ect due to the exposure, a winding wound about saidcore, and acondenser connected to the Winding, the condenser and the winding forming a circuit whichis resonant at the frequency of thevoltage effect induced from the power circuits;

2. An arrangement for protecting the conductors enclosed within a cable sheath from the infiuence of induction from a power circuit to which the sheath and its enclosed conductors are exposed, comprising means for grounding the cable sheath at the ends-of the exposed section, a core of iron composed of two semi-circular 'parts which may be mounted about the exposed sheath substantially at the center of the exposure, gaps existing between the two semi-circular parts, a coil of wire woundabout said iron core, and a condenser connected to terminalsof said coil, said coil and condenser being tuned to the frequency of the current induced from the power circuits.

' 3. Apparatus for suppressing the inductive interference induced into the conductors enclosed" within a cable sheath exposed to a power circuit or the like comprising a plurality of sections each composed of a magnetic core of iron formed by two semi-circular sections spaced from each other by air gaps and coaxial .with'the cable sheath, the cable sheath being grounded at-both ends of each section of the exposed area, a core of wire wound about the iron core, and a condenser connected to the terminals of the coil, said coil and condenser being resonant to the frequency of the voltage cf fect introduced into'the conductors due to the exposure. r 4. An arrangement for suppressing the voltage effect of a power circuit induced into the conductors enclosed within a cable sheath which is exposed to the power circuit, comprising a core of iron located about the cable sheath, a coil of wire the ends of which are connected to two points along the cable sheath which are on opposite sides of the iron core, and a condenser coupled to said coil of wire, the condenser and the circuit to which it is coupled being anti-resonant at thefrequency of the voltage induced from the power circuit.

5. Means for suppressing the voltage induced from a power circuit into the conductors enclosed within a cable sheath which is exposed to the power circuit, comprising a core of iron which is positioned about the cable sheath, said core of iron having 3.11 air gap, a transformer the primary winding of which is connected to two points along the cable sheath which are located on 'opposite'sides of'the iron core, and a condenser connected to thesecondary winding of the transformer said condenser and the circuit to. which itis coupled being anti-resonant at the frequency of the voltage induced from the power circuit into the various conductors.

6. An arrangement for suppressing the voltage of a predetermined frequency inductively introduced into the conductors enclosed within a cable sheath which is exposed to a power circuit or the like, comprising a core of iron composed of two semi-circular parts which are positioned about the cable sheath although spaced from each other by an air gap, a coil of wire coupled to that portion of the cable sheath which is enclosed by the iron core, and a condenser coupled to said coil of wire, the condenser and the circuits to which it is coupled being antiresonant at the frequency of the induced voltage.

7. A cable sheath enclosing a plurality ofconductors exposed to a circuit which induces a voltage of a predetermined frequency in said conductors, a core of iron composed of two semicircular sections, the two sections of said core being positioned about the cable sheath which houses said conductors although spaced from each other by air gaps, the various conductors being insulated from each other and from the cable sheath, a coil of wire wound about said core of iron, and a condenser connected to the terminals of the coil of wire, whereby the voltage induced into the various conductors enclosed within the cable sheathwill be neutralized by the voltage set up by the coil of wire and the condenser to which it is connected.

8. Means for eliminating the voltage effect of induction produced by a power circuit in the conductors enclosed within a cable sheath, con

sisting of means for grounding the cable sheath at the ends of the exposed section, a core of iron composed of two semi-circular parts which may be mounted about the exposed sheath substantially at the center of the exposure, gaps existing between the two semi-circular parts, and a condenser the two terminals of which are connected respectively to the cable sheath at two points on the opposite sides of the iron core, said condenser and the cable sheath circuit between said two points having a natural frequency substantially equal to the frequency of the interfering induction.

9. Means for eliminating the voltage efiect of induction produced in the conductors enclosed within a cable sheath which is exposed to a power circuit, a section of the exposed cable sheath being grounded at both ends, an iron core surrounding a part of the grounded section of the cable sheath, and a condenser coupled to the grounded section of the cable sheath, the condenser and the grounded section of the cable sheath together forming a circuit which is antiresonant at the frequency of the induced voltage effect.

LAURICE L. LOCKROW. 

